North American river otters have lived for a long time in Chesapeake Bay, yet relatively little is known about how their surroundings impact them. So what does daily life for river otters on the Atlantic coast of the US look like? What do they eat? Where do they socialize? Where do they go to poo? Researchers in Maryland decided to investigate and have now published their findings in Frontiers in Mammal Science.
"River otters in the Chesapeake Bay eat a wide range of animals, including those that live in the water and on land. Parasites, too, are crucial parts of their diet," said senior author Dr Katrina Lohan, a parasite ecologist and head of the Coastal Disease Ecology Laboratory at the Smithsonian Environmental Research Center (SERC). "We also found that river otters use manmade structures for latrines or choose areas with specific characteristics."
Poo on the dock of the bay
"We started this research when a colleague sent me an email about some weird-looking, watery poo that she found at the dock on our campus. In the poo was a fire engine red worm," Lohan said. Eventually wildlife cameras caught a river otter pooing on the dock. Since the worm was most likely a parasite, Lohan decided to get some samples. The article's first author Calli Wise, a graduate researcher at the SERC, collected further samples from 18 active otter latrines. Over 11 months, the researchers recovered 28 scats per latrine on average.
"Scats usually smelled strongly of fish and were full of scales or crustacean shells," Wise said. "We wore gloves and used sterile tools and tubes to collect scat samples, to avoid contamination with other DNA."
Latrines are central hubs for otters: While they are primarily used for scent-marking through defecation and urination, river otters also use them as spots to eat, play, socialize, and rest. Typically, they look like areas where the grass or other vegetation has been flattened with low overhanding vegetation for cover.
Most latrines on the Chesapeake Bay were located close to the shoreline, in areas with woody debris and open access to the water. Often, hard to miss features made them stand out from the surrounding landscape. Such features included man-made structures and five otter latrines were found on docks, boardwalks, and shoreline staircases.
Parasite control
The collected scat was taken to the lab for analysis. The team used two methods to identify what the otters had eaten: metabarcoding, a method that uses genetic information, and microscopic examination.
"We collected most scats in the summer, when river otter diet is likely diverse," Wise said. "We found evidence of invasive species like common carp and white river crayfish in river otters' diet." In addition, otters ate more fish and crustaceans, such as American blue crabs. They also included the occasional duck or amphibian on their meal plan.
Otter poo, however, didn't only tell the researchers what the otters had eaten. Using metabarcoding, they were able to determine what parasites were in the prey. They then matched parasites to the hosts they were most likely to infect. Many of the parasites are known to infect teleost fish, an extremely diverse group of ray-finned fishes that are the primary prey of river otters But a few other parasites likely infected otters directly. "It is possible that river otters, like other top predators, wouldn't be able to find enough food to eat without parasites," Lohan pointed out.
The team said that they weren't able to clearly identify all parasites, mainly because of missing sequences in databases that new sequences might be matched to. They also could not identify individual diets or match poo samples to individual otters.
Nevertheless, the presence of parasites in otter poo might mean river otters are important ecosystem engineers. "Since so many of the parasites are actually infecting otters' prey, it could mean that river otters are culling sick individuals from the populations they are preying upon," Lohan explained. This could potentially alter evolutionary processes for their prey, since infected individuals, once eaten, no longer contribute to the gene pool.
"Some of the parasites that infect river otters could potentially also infect humans, who also are mammals," Lohan concluded. "Thus, we could use river otters as 'disease sentinels,' and study them to learn about what public health threats occur in certain areas."
